Coherent optical detection was considered intensively for fiber optics in the 80's and 90's. However, with the advent of the optical amplifier the work on this versatile and highly sensitive receiver technique was put aside. In the meantime great improvements on the optical components side have been achieved. These include laser output power, linewidth stability and noise as well as the bandwidth, power handling capability, and common mode rejection of (balanced) photo detectors. The advances in electronic microwave components improved in a way that the advantages of optical coherent detection over direct detection can be used. The present inventors recognize that this makes coherent optical detection very attractive for future communication links.
For free space optical communication links coherent detection was always of interest since it has to rely on high power laser and sensitive receiver techniques. One application is the optical satellite link which can equal or exceed the data throughput of an entire suite of microwave transponders. Moreover, the optical system has a much tighter beam size than all RF systems, making it intrinsically more secure. With the possibility of entire suites of RF transponders being replaced by a single optical communication system, the level of complexity of spacecraft (SC), and their associated weight and power consumption, all decrease.
Optical coherent detection offers a number of advantages over conventional intensity modulation/direct detection. For example, the use of an optical coherent receiver allows detecting signals of very low strength, lower than that of conventional optical receivers. This is especially important in order to detect signals at optical wavelengths where low noise optical amplification is not available. Moreover, coherent detection allows preserving the phase information of the optical signal. This is useful to detect optical signals where the information is comprised in the phase of the electromagnetic wave. This requires a stable phase and/or frequency lock between the received optical signal and the optical local oscillator used in the coherent receiver.
In the coherent receiver structure the received optical signal is mixed with the light of an optical local oscillator (LO). In this way, the signal is down converted from the optical carrier frequency (˜200 THz at 1.55 μm) to a microwave carrier frequency (typically a few gigahertz). The resulting beat signal after photo detection exhibits a center frequency that corresponds to the intermediate frequency fIF(IF), which is the difference between the signal frequency and the LO frequency.
If the signal frequency and the LO frequency are the same, the detection technique is called “homodyne.” For different center frequencies of signal and LO, the system is referred to as “heterodyne,” with fIF=fc−fLO, where fc and fLO are the center frequencies of the received signal and LO, respectively. For heterodyne systems, the IF has to be at least two times the data rate of the optical signal to receive the double-sided data spectrum. Homodyne reception requires that the LO, typically produced by a laser, be phase locked to the incoming optical signal, whereas heterodyne detection calls for frequency locking to the received signal.
In many transmission scenarios, homodyne systems can provides higher sensitivity than heterodyne systems. Homodyne detection requires an RF bandwidth approximately equal to the transmitted data rate, whereas heterodyne detection requires an RF bandwidth approximately equal to two to three times of the transmitted data rate. From purely a bandwidth perspective, homodyne is less demanding than heterodyne detection. However, homodyne detection is more demanding in its implementation compared to heterodyne, mainly because of homodyne's strict requirement for phase locking.
The main building blocks of a coherent receiver include an optical local oscillator, optical coupler, balanced photodetector, phase/frequency locking, polarization control loop, and electrical signal processing. Within these blocks, there are several requirements necessary to achieve high receiver sensitivity:                1. A high-power, optical local oscillator with low relative intensity noise (RIN), low laser linewidth, and high optical isolation.        2. Polarization matching between the signal and LO laser.        3. Optical mixer with 50/50 coupling ratio on output ports.        4. Optical path length equalization into the balanced photodetector.        5. A balanced photodetector with high responsivity, high optical-power-handling capacity, and good common-mode rejection ratio (CMRR).        6. Phase/frequency locking to reduce the phase and frequency noise of the IF.        